The present invention relates generally to attitude control systems for a spacecraft, and more particularly, to a method and apparatus for counteracting a disturbance in a spacecraft.
As a spacecraft orbits the earth, the position of the spacecraft relative to the earth must be periodically corrected by firing thrusters to maintain the spacecraft in the desired orbit. During an orbit correction, the spacecraft's attitude must be maintained in the desired direction. Oftentimes a reaction wheel system or gimbaled momentum wheel is used to provide the desired torque to maintain the spacecraft's attitude. As a result of opposing disturbing torques, for example solar radiation pressure, the momentum builds up within the momentum wheel making it necessary to dump momentum before the momentum wheel reaches its maximum speed. This is commonly done by firing spacecraft on-off thrusters.
The use of thrusters for momentum dumping uses valuable propellant. Satellite life and hence satellite revenue is often propellant limited. Thus, the propellant consumed for thruster momentum dumping reduces potential revenue.
It is known that thruster efficiencies increase with the average duration of thruster pulses. Thus, the longer the burn times of the thruster, the higher the efficiency of the burn process. This utility has motivated many prior art methods for increasing the size of thruster pulses used for momentum dumping while keeping the amplitude of the resulting attitude transient below a given bound.
The prior art is described for an idealized momentum dumping example. A roll thruster angular momentum impulse unloads a roll wheel which has 0.15 N-m of available torque. The satellite roll inertia is 1000 kg-m^2. The allowable thruster impulse is that such that the angular excursion from the nominal is no more than 1 milliradian. All quantities are perfectly known.
In the following, “one-sided transients” denote those where the roll errors are predominantly of one sign, and “two-sided transients” denote those where significant roll errors of both signs are seen.
In a first prior art method, closed loop feedback of roll attitude error to roll wheel torque is used. After the roll thruster impulse, an opposing wheel torque develops, eventually slowing the spacecraft down and returning it to its nominal attitude. The resultant excursion is predominantly one-sided, in the direction produced by the thruster impulse. This method does not rely on a priori knowledge of the sign, timing or magnitude of the thruster impulse. The allowable thruster impulse here is determined by how quickly and forcefully the control system responds. The allowable thruster impulse is less than 0.55 N-m-s.
In a second prior art method, feedforward is used. At the application of the roll thruster impulse, an opposing full torque roll wheel command is applied simultaneously. The duration of the wheel torque command is set so the total wheel momentum impulse is equal and opposite that of the thruster impulse. At the end of the impulse, the attitude is offset, but can be returned to nominal by means such as closed loop wheel control. The resultant excursion is predominantly one-sided, in the direction produced by the thruster impulse. This method relies on a prior knowledge of the sign, timing and magnitude of the thruster impulse. The allowable thruster impulse is 0.55 N-m-s.
In a third prior art method, prebias is used. The spacecraft is biased in roll angle, approximately 50% of the magnitude, in the opposite direction of the expected transient. At the end of the impulse, the attitude is offset, but can be returned to nominal by means such as closed loop wheel control. Wheel control may be by the first or second prior art method, or a combination. The resultant excursion is two-sided. This method relies on a priori knowledge of at least the sign of the thruster impulse. The allowable thruster impulse is 0.77 N-m-s for the feedforward variant.
In a fourth prior art method, pre-emphasis torque is used. This is an anticipatory feedforward technique. An open loop wheel torque pulse is initiated in advance of the thruster pulse, such that the thruster pulse will be centered on the wheel torque pulse, and the total wheel momentum impulse is equal and opposite that of the thruster impulse. The resultant excursion is predominantly one-sided, in the direction produced by the thruster impulse. The nominal attitude is reached at the end of the wheel torque pulse. This method relies on a priori knowledge of the sign, timing and magnitude of the thruster impulse. The allowable thruster impulse is 1.10 N-m-s.
A fifth prior art method follows that of the fourth prior art method, except that the timing is chosen so that the thruster impulse occurs at a time which is (sqrt(2)−1)=41.4% of the way through the wheel torque pulse, rather than 50% in the fourth prior art method. The resultant excursion is two-sided, with equal amplitude error on both sides. At the end of the impulse, the attitude is offset, but can be returned to nominal by closed loop wheel control or equivalent means. This method relies on a priori knowledge of the sign, timing and magnitude of the thruster impulse. The allowable thruster impulse is 1.32 N-m-s.
A sixth prior art method is described in U.S. Pat. No. 6,439,509. The system provides a method for manipulating the states of the spacecraft's nutation compensator. The system is used to reset one of the states in the nutation compensator polynomial to a value in proportion to the magnitude of the expected roll momentum unload so as to develop a short term transient that is equal and opposite to that of the roll unload. Thus, the compensator will automatically damp out the roll unload transient without biasing the spacecraft in roll. The timing can be critical and is best found by empirical means, or heuristically. Since this method is explicitly presented as a replacement for the third prior art method, the transient is predominantly one-sided. The allowable thruster impulse will be a function of the polynomial used, but should be less than 1.10 N-m-s, since that is the best of the other one-sided methods.